Low vacuum vapor process for producing superconductor articles with epitaxial layers

ABSTRACT

A method for fabricating superconductor articles with an epitaxial layer is described. The method can be performed under conditions of relatively high pressure and low substrate surface temperature. The resulting epitaxial layers can demonstrate various advantageous features, including low pore density and/or inclusions with small average particle size diameter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/059,604, filed Sep. 23, 1997.

BACKGROUND OF THE INVENTION

The invention relates to methods of making superconductors havingepitaxial layers.

Superconductors are used in a variety of applications. Often, themechanical integrity of a superconductor can be enhanced by forming amultilayer article that includes a layer of superconductor material anda substrate layer, but the use of a substrate can present certaincomplications.

Chemical species within the substrate may be able to diffuse into thelayer of superconductor material, potentially resulting in a loss ofsuperconductivity. Moreover, the coefficients of thermal expansion aswell as the crystallographic spacing and orientation of the substrateand the superconductor layer can be different, causing the article topeel apart during use.

To minimize these complications, a buffer layer can be disposed betweenthe substrate and the superconductor layer. The buffer layer shouldreduce diffusion of chemical species between the substrate and thesuperconductor layer, and the buffer layer should have a thermalcoefficient of expansion that is about the same as both the substrateand the superconductor layer. In addition, the buffer layer shouldprovide a good crystallographic match between the substrate and thesuperconductor.

One approach to controlling the crystallographic properties of a layerhas been to use epitaxy. An epitaxial layer is a layer of material thatis grown on the surface of a substrate such that the crystallographicorientation of the layer of material is determined by the latticestructure of the substrate. By epitaxy is also meant materials withordered surfaces whether formed by conventional epitaxy orgraphoepitaxy. Epitaxial layers have been grown using physical vapordeposition (PVD), chemical vapor deposition (CVD) and sputteringtechniques.

Typically, PVD involves the evaporation of a solid material and transferof the vapor to the substrate surface in a diffuse gas beam in whichonly a small portion of the total amount of evaporated solid may reachthe substrate surface. Thus, the material usage efficiencies obtainedwith PVD can be low. In addition, PVD is usually performed at a chamberpressure of at most about 1×10⁻⁴ torr, so the flux of evaporated solidat the substrate surface can be small, resulting in low epitaxial layergrowth rates.

In CVD, one or more reactant gases within the chamber adsorb to thesubstrate surface and react to form the epitaxial layer with productgases desorbing from the substrate surface. Generally, the reactantgases reach the substrate surface by convection and diffusion, so thematerial usage efficiencies can be low. Furthermore, CVD is typicallyconducted at a chamber pressure of at least about 0.1 torr, and, to growepitaxial layers at these elevated pressures, relatively high substratetemperatures are usually used. Thus, the selection of substratematerials used in CVD can be limited.

Sputtering methods of growing epitaxial layers can also be limited bythe aforementioned considerations.

When using PVD, CVD or a sputtering technique, the quality of theepitaxial layer can depend upon the chemical nature of the substratesurface during layer growth. For example, contaminants present at thesubstrate surface can interfere with epitaxial layer growth. Inaddition, native oxides present at the substrate surface can help orhinder epitaxial layer growth. Further, PVD, CVD and sputtering methodscan be ineffective at providing control of the chemical nature of thesubstrate surface during layer growth, so the epitaxial layers formed bythese techniques can be of poor quality.

SUMMARY OF THE INVENTION

The invention features a low vacuum vapor deposition process forproducing a superconductor article with an epitaxial layer. Theepitaxial layer is disposed on a substrate that can be of non-identicalcomposition.

In one aspect the invention features a method that includes the steps ofplacing a textured or crystallographically oriented target surface of asubstrate, typically including contaminant materials, in a low vacuumenvironment, and heating the target surface (substrate surface) to atemperature which is greater than the threshold temperature for formingan epitaxial layer of the desired material on the substrate material ina high vacuum environment under otherwise comparable conditions. Alayer-forming stream, including an inert carrier gas and a dispersion ofa first species (layer forming gas), which is a chemical component ofthe desired epitaxial layer, is directed at a positive velocity greaterthan about 1 m/sec toward the substrate surface through the low vacuumenvironment. A second species (conditioning gas) is provided in the lowvacuum environment, directed toward the substrate surface at a velocitysubstantially similar to the velocity of the layer-forming stream, andreacted with one or more of the species present in the substratesurface, for example, a contaminant material. This reaction conditionsthe substrate surface and promotes nucleation of the epitaxial layer. Adesired material chemically comprising the first layer forming gas isdeposited from the stream onto the substrate surface to form anepitaxial layer. A layer of an oxide superconductor is then deposited onthe epitaxial layer.

As used herein, "layer forming gas" refers to a gas that can adsorb to asurface and become a component of an epitaxial layer. A layer forminggas can be formed of atoms, molecules, ions, molecular fragments, freeradicals, atomic clusters and the like.

As used herein, "conditioning gas" refers to a gas that can interactwith a surface to remove surface contaminants, remove undesired nativeoxides present at the substrate surface or form desired native oxides orother components at the substrate surface. A conditioning gas can beformed of atoms, molecules, ions, molecular fragments, free radicals,atomic clusters and the like.

In another aspect, the invention features a method of making asuperconductor. The method includes growing, at a chamber pressure of atleast about 1×10⁻³ torr, an epitaxial buffer layer on a substrate havinga temperature that is about the same as a PVD epitaxial growth thresholdtemperature for a chamber pressure of at most about 1×10⁻⁴ torr. Themethod also includes depositing a superconductor material or a precursorof a superconductor material on the epitaxial buffer layer.

A "precursor of a superconductor material" as used herein refers to amaterial which can undergo subsequent treatment to become asuperconductor. For example, subsequent to heating a particulartemperature, a precursor of a superconductor material can become asuperconductor material.

For a given epitaxial layer, the "PVD epitaxial growth thresholdtemperature for a chamber pressure of at most about 1'10⁻⁴ torr" refersto the minimum substrate temperature that can be used to grow theepitaxial layer at a chamber pressure of at most about 1×10⁻⁴ torr,typically at most about 1×10⁻⁵ torr, using a PVD with a diffuse gasbeam.

As used herein, a "diffuse gas beam" refers to a gas beam in which lessthan about 50% of any layer forming gas in the gas beam is incident atthe substrate surface.

In still another aspect, the invention features a method of making asuperconductor. The method includes growing an epitaxial buffer layer ona substrate surface at a rate of at least about 50 Angstroms per secondin a vacuum chamber having a pressure of at least about 1×10⁻³ torr. Themethod also includes depositing a layer of superconductor material or aprecursor of a superconductor material on the epitaxial buffer layer.

In a further aspect, the invention features a method of making asuperconductor. The method includes growing an epitaxial buffer layer ona substrate surface by exposing the substrate surface to a gas beamhaving a layer forming gas, wherein at least about 75% of the layerforming gas in the gas beam is incident at the substrate surface. Themethod also includes depositing a layer of superconductor material or aprecursor of a superconductor material on the epitaxial buffer layer.

In still a further aspect, the invention features a method of making asuperconductor. The method includes forming an epitaxial buffer layer byexposing a surface of a substrate to a conditioning gas that interactswith the substrate surface to form a conditioned substrate surface andexposing the conditioned substrate surface to a gas beam having a layerforming gas that becomes a component of an epitaxial buffer layer on theconditioned substrate surface. The exposing steps are performed at apressure of at least about 1×10⁻³ torr. The method further includesdepositing a layer of a superconductor material or a precursor of asuperconductor material on the buffer layer.

A "conditioned surface" herein denotes a surface from which asubstantial portion of surface contaminants or undesired native oxideshave been removed by a conditioning gas, or on which a desired nativeoxide or other compounds has been formed by a conditioning gas.

In a still further aspect, the invention relates to a method of making asuperconductor. The method includes growing an epitaxial superconductorlayer on a surface of a material having a temperature that is about thesame as a PVD epitaxial growth threshold temperature for a chamberpressure of about 1×10⁻⁴ torr. The growing step is performed in a vacuumchamber having a pressure of at least about 1×10⁻³ torr, and thematerial can be a buffer layer or a single crystal.

In one aspect, the invention features an article that includes anepitaxial buffer layer and a superconductor layer disposed on the bufferlayer. The buffer layer has a pore density of less than about 500 poresper square millimeter.

As used herein, "pore density" refers to the number of pores at thesurface of an epitaxial layer per unit area of the surface of theepitaxial layer.

In another aspect, the invention features an article that includes anepitaxial buffer layer and a superconductor layer disposed on the bufferlayer. A surface of the buffer layer has inclusions with an averageparticle size diameter less than about 1.5 micrometers.

As used herein, "inclusions" refer to surface defects, such as secondphase particles, that can act as locations of initiation ofnon-epitaxial growth at a surface.

The substrate material may be ceramaceous, such as an oxide, metallic,such as a metal or alloy, or intermetallic. It chemically comprises oneor more substrate species, and at least one substrate species isdifferent from the layer forming gas. In addition to the substratespecies, the species present in the substrate surface may include nativeoxides of one or more of the substrate species, and adsorbed surfacecontaminants.

Since the epitaxial layer typically includes a metal, the layer forminggas is usually a metal vapor. If the epitaxial layer includes more thanone species, such as a metal oxide, the layer forming gas can comprisemore than one gas. For example, if the epitaxial layer is formed ofcerium oxide, the cerium vapor and oxygen can be used in the gas beam.

The conditioning of a substrate surface can occur by exposing thesubstrate surface to a background pressure of a conditioning gas.Alternatively, the conditioning of a substrate surface can occur byexposing the surface to a gas beam that includes a conditioning gas.Such gas beams can be directed gas beams or diffuse gas beams, and theconditioning gas included therein may have a high velocity or a lowvelocity. In some embodiments, conditioning of the substrate can occurby exposing the substrate surface to a background pressure of aconditioning gas and a conditioning gas that is included in the gasbeam.

When using a background pressure of a conditioning gas to condition asubstrate surface, the vacuum chamber typically has a partial pressureof the conditioning gas of at least about 0.5 percent, preferably fromabout 0.5 percent to about 10 percent, and more preferably from about 2percent to about 6 percent. Thus, for example, if the vacuum pressurewere about 100 millitorr, the partial pressure of hydrogen in thechamber can be at least about 0.5 millitorr.

In embodiments in which a conditioning gas is included in a gas beam,the conditioning gas may be introduced in the same stream as the layerforming gas or in a separate stream. In some embodiments, other speciesmay also be introduced which are reactive either with the layer forminggas or with one or more additional species present in the substratesurface, but the stream containing the layer forming gas preferablycontains only one other reactive species. Additional reactive speciesare preferably introduced in separate streams of substantially similarvelocity. Large dissimilarities in stream velocities can favor one setof species reactions at the expense of the others, and are generally tobe avoided. The desired epitaxial layer material may be the layerforming gas or the reaction product of the layer forming gas with one ormore of the other species introduced into the low vacuum environment.The desired material may be ceramaceous, metallic, or intermetallic.Moreover, the desired material can be an oxide superconductor.

The substrate surface can be exposed to the conditioning gas and thelayer forming gas in series or in parallel. For parallel processes, oneportion of the substrate surface is conditioned while the epitaxiallayer is grown on another portion of the substrate surface.

The chemical nature of the conditioning gas depends upon species presentat the surface on which the epitaxial layer is grown. For example, ifthe surface has an undesired native oxide present at its surface, theconditioning gas can be a reducing gas, such as hydrogen. Alternatively,the conditioning gas can be an oxidizing gas, such as oxygen, if it isdesirable to form a native oxide at the surface or if sulfur or carbonare contaminants present at the surface.

The velocities of the layer forming gas, conditioning gas and additionalgaseous species dispersed in their streams should be positive, that isdirected toward the substrate surface. In some embodiments, thevelocities are greater than about 1 m/sec. Velocities as high assupersonic may be used. Velocities in the range of from about 10 m/secto about 400 m/sec are preferred at typical low vacuum conditions. In apreferred embodiment, the velocity of the layer forming gas isoriginally high and is reduced from a high velocity to a low velocityprior to contact with the substrate surface, which in essence reducesits kinetic energy. The reduction in velocity is accomplished byinterference from the low vacuum environment and also from a boundarylayer of carrier gas which forms at the substrate surface. The decreasein the kinetic energy assists in thermal equilibration of the desiredmaterial with the substrate. While the high initial velocity of thegaseous species aids in the efficient transport of the layer forming gasto the substrate surface and thereby facilitates higher depositionrates, low velocity gases may also be effectively used in the scope ofthis invention. The term high velocity refers to positive velocitieswhich approach at least about sonic levels, i.e the speed of sound atoperating vacuum levels, such as 100-400 m/sec. The term low velocityrefers to positive velocities of less than 10 m/sec and more generallyless than 100 m/sec, but greater than 1 m/sec.

The term elevated temperature(s) refers to temperatures which aresufficient to allow diffusion of the desired material to low energyequilibrium sites after arrival at the substrate surface. The range ofelevated temperatures will vary depending upon the materials involvedand other specifics of the deposition process, such as vacuum level anddeposition rate. Although this is a low vacuum process, the lowertemperature limit is set as the threshold temperature for forming anepitaxial layer of the desired material on the substrate material in ahigh vacuum environment at the predetermined deposition rate. As usedherein, the term low vacuum (or high pressure) refers to a vacuumpressure which is achievable using mechanical pumping systems or apressure greater than or equal to about 1×10⁻³ torr, and the term highvacuum (or low pressure) refers to pressure less than about 1×10⁻⁵ torrof base vacuum. Temperatures found suitable for forming epitaxial layersin the invention correspond to temperatures that are suitable forforming epitaxial layers with pressures that are from about 10³ to 10⁶lower, and preferably from about 10³ to 10⁴ lower, than pressures usedwith methods that involve diffuse gas beams. Thus, for example if apressure of about 1×10⁻¹ torr is used in the invention to form anepitaxial layer of a desired material on a given substrate material at ain particular growth rate, the substrate temperature can be the same aswould be used at a pressure of from 1×10⁻¹ to 1×10⁻⁷ torr to grow thesame epitaxial layer under the same conditions with a diffuse gas beam.An upper end of the elevated temperature range is set as 90% of themelting point of the selected substrate material. Typically thesubstrate surface is heated to a temperature which is less than theprior art threshold temperature for forming an epitaxial layer of thedesired material on the substrate material in the low vacuum environmentat the predetermined deposition rate.

In some embodiments, an additional conditioning gas which can bereactive with the layer forming gas is provided. In other embodiments,the conditioning gas is reactive with the layer forming gas as well asone or more species present at the substrate surface. In embodiments inwhich the substrate material is metallic and the epitaxial layer is anoxide of the layer forming gas, the conditioning gas may be chosen to bemore oxidizing with respect to the layer forming gas than with respectto a native oxide of the substrate material. In embodiments in which thesubstrate and epitaxial layer are both metallic, the conditioning gasmay be reducing with respect to both. In some embodiments, a layerforming gas can also serve as a conditioning gas. For example, oxygencan be used to remove impurities and/or an oxide of the substratesurface as well as form an oxide epitaxial layer.

When a substrate of preferred crystallographic orientation is used, suchas a cubic textured substrate, deposition of an epitaxial layer onto thesubstrate according to the process of this invention provides acomposite material which is suitable for preparing a superconductorcomposite. The cubic texture of the substrate is transferred to theepitaxial layer which, in turn may transfer this orientation to asuperconducting oxide layer. The superconducting oxide layer may bedeposited onto the substrate and epitaxial layer using the process ofthis invention.

The invention allows the application of high vacuum-like evaporationprocesses to the fabrication of large scale substrate areas withoutlimitations which are typically imposed by a high vacuum system.Material usage efficiencies are typically increased from less than 50percent to over 95 percent by the directed layer-forming depositionstreams. Furthermore, the directed layer-forming stream in the lowvacuum environment reduces the uncertainty associated with the formationof epitaxial films on metallic substrates. The deposition stream canproduce a surface boundary layer which enhances the nucleation andgrowth of epitaxial films on the substrate surface and reduces thetemperatures required for epitaxial film growth. The addition of atleast one conditioning gas, as allowed by the low vacuum operation, canbe used to disrupt a native surface oxide on substrate materials whichallows the template of the crystallographically oriented substrate,preferably a cubic textured alloy, to be directly available for thedepositing atoms of the preselected material. The layer forming gas andthe conditioning gas can be selected to thermodynamically favor specificcompounds and materials to improve epitaxial film growth. A consistentprocess is developed as well, which can eliminate the system to systemvariation in the deposition of epitaxial films in high vacuum systems.The high energy, low vacuum approach of the present invention alsoprovides the opportunity to use a single manufacturing method for boththe buffer layer and the superconducting film layer of a compositearticle without high vacuum processing.

For epitaxial layers, it is generally disadvantageous to have a highpore density or inclusions with a large average particle size. Theinvention can provide epitaxial layers having low pore densities and/orinclusions with small particle sizes. The epitaxial layers preferablyhave a pore density of less than about 500 pores per square millimeter,more preferably less than about 250 pores per square millimeter, andmost preferably less than about 130 pores per square millimeter. Theepitaxial layers preferably have inclusions with an average particlesize of less than about 1.5 micrometers, more preferably less than about1 micrometer, and most preferably less than about 0.5 micrometers.

In embodiments in which a conditioning gas is included in a gas beam,the gas beam which impinges upon the substrate surface should include asufficient amount of conditioning gas to condition the substrate surfaceand allow the epitaxial layer to grow. Preferably, the gas beam thatimpinges upon the substrate surface includes from about 0.5 molarpercent to about 10 molar percent conditioning gas, more preferably fromabout 1 molar percent to about 8 molar percent conditioning gas, andmost preferably form about 2 molar percent to about 6 molar percentconditioning gas.

In some embodiments, it can be advantageous to form the epitaxial layerat a fast rate. In these embodiments, the epitaxial layer is preferablygrown at a rate of at least about 50 Angstroms per second, morepreferably at least about 100 Angstroms per second, and most preferablyat least about 150 Angstroms per second. These growth rates can beachieved at a vacuum chamber pressure of at least about 1×10⁻³ torr.

The epitaxial layers can be formed without using electron beams or ionbeams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 outlines the low vacuum vapor process to produce epitaxiallayers;

FIG. 2 illustrates an embodiment of an apparatus that can be used withthe method of the invention;

FIG. 3 illustrates a boundary layer formed on a substrate;

FIG. 4 is a close-up view of a portion of FIG. 3 illustrating the motionof individual atoms in the boundary layer;

FIGS. 5A and 5B illustrate multistream vapor deposition processes inaccordance with the invention;

FIG. 6 illustrates a superconductor article fabricated by a low vacuumvapor deposition process;

FIG. 7 illustrates another embodiment of an apparatus that can be usedwith the method of the invention;

FIG. 8 is an x-ray diffraction pole figure of cerium oxide deposited onsingle crystal nickel under conditions where uniformly oriented epitaxydoes not occur, as seen in Example II;

FIG. 9 is an x-ray diffraction pole figure of cerium oxide deposited oncrystallographically textured nickel under conditions where epitaxyoccurs, as seen in Example IV;

FIGS. 10A-10C are x-ray diffraction pole figures of a nickel substrate,the nickel substrate having a cerium oxide epitaxial layer disposedthereon, and the nickel substrate/cerium oxide layers having anepitaxial yttria-stabilized zirconia layer disposed thereon; and

FIG. 11 illustrates the transition temperature of the superconductor ofExample 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a method of fabricating a superconductor articlewhich has an epitaxial layer. A preferred embodiment of the method isoutlined in FIG. 1 and includes placing a textured substrate surface ina low vacuum environment 101, and heating the substrate surface to anelevated temperature via a heat source. In addition to the substratespecies, the species present in the substrate surface typically includecertain contaminant materials such as undesired native oxides of one ormore of the substrate species and adsorbed surface contaminants. In someembodiments, desired native oxides of one or more of the substratespecies may be present. A stream including a layer forming gas, aconditioning gas and an inert carrier gas is directed at high velocitytoward the substrate surface through the low vacuum environment 102.Alternatively, step 102 is performed by providing a background pressureof the conditioning gas and a stream including a layer forming gas and acarrier gas is directed at the substrate surface. The conditioning gasreacts with one or more of the species present in the substrate surfaceto condition the substrate surface and promote nucleation of theepitaxial layer 104. In the preferred embodiment illustrated, thevelocity of the layer forming gas is reduced from high velocity to lowvelocity 103 by interference with the substrate, a gaseous boundarylayer adjacent the substrate surface and the low vacuum environment. Thelayer forming gas, or in some embodiments, its reaction product with theconditioning gas, is deposited onto the substrate at an elevatedtemperature to form an epitaxial layer 105. Oxide superconductors aredeposited onto the epitaxial layer to form a superconducting compositearticle.

The substrate surface is heated to an elevated temperature which is lessthan about 90% of the melting point of the substrate material butgreater than the threshold temperature for forming an epitaxial layer ofthe desired material on the substrate material in a high vacuumenvironment at the predetermined deposition rate. Typically thesubstrate surface is heated to a temperature which is in less than theprior art threshold temperature for forming an epitaxial layer of thedesired material on the substrate material in the low vacuum environmentat the predetermined deposition rate. Temperatures suitable for formingepitaxial layers in vacuums from about 10³ to 10⁶ lower, and preferablyfrom about 10³ to 10⁴ lower, than the actual vacuum conditions may beused. Epitaxial metal layers have successfully been produced attemperatures of 77° K. and below. Typical lower limit temperatures forthe growth of oxide layers on metal are approximately 200 degreesCelsius. The growth of epitaxial oxide films on oxide substrates canoccur at lower temperatures. In some embodiments, the substrate surfacehas a temperature of from 200° C. to 800° C., preferably 500° C. to 800°C., and more preferably from 650° C. to 800° C. Various well-knownmethods such as convection heating and conduction heating may be used toobtain the desired temperature elevation.

Referring to FIGS. 2-4, there is shown a directed vapor depositionmachine for use, for example, with the low vacuum process of the presentinvention. Other apparatus for forming and directing a vapor stream mayalso be used. The machine 10 includes an evaporation housing 20 for ahigh pressure environment which includes a vapor source 21 for a layerforming gas 22 and a source 23 for a conditioning gas 46 and inertcarrier gas 26 to form a dispersion which is ejected from the housing 20by a nozzle 28 through a nozzle opening 30 at high velocity and energy.If the conditioning gas is provided in machine 10 at a backgroundpressure, machine 10 can include a leak valve for entering theconditioning gas into machine 10. Such leak valves are well known tothose skilled in the art.

The dispersion forms a high speed stream 34 of the carrier gas 26, layerforming gas 22 and conditioning gas 46. This stream is usually adirected gas beam. The stream 34 is directed by the source 28 at nearsonic velocity, such as 100-400 m/sec, through a low vacuum environment32 toward the substrate surface 42 of a work piece or substrate 40. Thevelocity of the layer forming gas 22 is reduced from the high energy andhigh velocity of the stream to low energy and velocity as the individualatoms 36 of the layer forming gas 22 are deposited onto the surface 42of the substrate 40. This is accomplished through the collision of theatoms or molecules of the layer forming gas with the gas molecules in agaseous boundary layer which is created by the impingement of thecarrier gas 26 as the carrier gas 26 strikes the substrate surface 42.The result is that the individual atoms or molecules 36 contact thesubstrate 40 at low kinetic energy and low velocity, thereby settlinginto a desirable and stable low energy configuration for epitaxial layernucleation and growth. This forms the deposited layer 44 on substrate40.

The directed flow of gas from nozzle opening 30 can result in highmaterial usage efficiencies. Preferably, at least about 75% of the layerforming gas that emanates from nozzle opening 30 is incident at thesubstrate surface, more preferably at least about 90%, and mostpreferably at least about 95%.

With the present system, one or more substrates 40 can either be heldstationary, or moved under computer control in an X-Y coordinate systemas shown by arrows 41 and 43 in FIG. 2. Movement of the substratefacilitates the use of multiple directed streams, as further illustratedand discussed in connection with FIGS. 5A and 5B. It can also be seenfrom FIGS. 2, 5A and 5B that a wide variety of materials may bedeposited to form different end products, such as metals, alloys,multilayers, composites, nitrides, oxides, or any other vapordepositable material.

With the present system, a substrate of unlimited length and finitewidth may be transported across the field of deposition of the directedvapor stream 34 as shown in FIG. 5A. A substrate 40 in the form of along tape may be mounted on reels external to the deposition chamber andpassed in to and out of the low vacuum environment through conventionalgas seals 75. As the substrate passes through the deposition chamber thesubstrate temperature is raised to the deposition temperature by theheaters 72. A directed vapor stream 34 impinges on the moving substrateboth controlling the substrate surface condition and depositing thelayer forming gas epitaxially upon the substrate surface.

As the substrate traverses the deposition chamber, additional directedvapor streams 35, 36, 37 can be used to either condition the substratesurface or deposit one or more epitaxial species on the substratesurface. Multiple nozzles 20, 21, 22, 23 can be employed in-line on anaxis coincident with the substrate transport direction to increase theamount of material which can be deposited in a given deposition chamberas shown in FIG. 5A. Alternatively, as shown in FIG. 5B, the nozzles 25,26, 27, 28 may be offset from the transport axis to allow broadercoverage of a wide strip as the substrate moves through the depositionchamber. Many alternative advantageous arrangements of the multipledeposition sources are readily apparent.

With the addition of the baffles 78 each source may produce a differentvapor composition with different reactive species and/or differentfunctions. For example, a first stream 37 may be used to cleanse thesubstrate of contaminants and native oxide scale. A second stream 36 maybe used to deposit a first epitaxial layer. A third stream 35 may beused to deposit a second epitaxial layer. A fourth stream 34 may be usedto deposit a superconducting oxide precursor or layer. Further epitaxiallayers can be added by additional gas beams. The low vacuum environmentallows these multiple depositions to be accomplished with simplebaffles, or gas seals between subchambers. If dissimilar directed vaporstreams are used, it is preferable to maintain the subchamber pressuressuch that the initial vapors are not contaminated by later vapors, i.e.the pressure in each subsequent chamber must be slightly lower than inprior chambers and all chamber pressures must be less than the sourcepressures. This can be accomplished by simple valving to differentiallypump the subchambers.

The vapor deposition system of the present invention 10 employs anevaporation source 20 which can operate under low inert gas pressure ofapproximately 2-5 torr. Conventional deposition sources, such assputtering, thermal, chemical or other sources such as thosecommercially available from Jet Process Corporation of New Haven, Conn.,are used to produce the vapor. Such sources are disclosed in, forexample, U.S. Pat. Nos. 5,265,205; 5,356,672; 5,356,673; and 5,571,332,each of which is hereby incorporated by reference.

The layer forming gas can be any material, with metallic and oxidesources preferred. Multiple evaporation sources and alloys may be usedconcurrently in a single stream for deposition of complex materials orsequentially in several streams for layered schemes. The layer forminggas should meet certain requirements, such as lattice match with thesuperconducting oxide and the substrate material, chemical compatibilitywith the substrate and the superconducting oxide, and appropriatethermo-physical and electrical properties depending upon variousapplications. Without intending any limitation, materials which may beused as epitaxial or buffer layers for a superconductor include CeO₂,yttria-stabilized zirconia, LaAlO₃, SrTiO₃, LaNiO₃, LaCuO₃, SrRuO₃,CaRuO₃, NdGaO₃, NdAlO₃.

The layer forming gas mixes with an inert carrier gas 26 which entersthe evaporation source 20 through opening 24. The carrier gas can bechosen and tailored to the requirements of the system, thereby allowinggreater freedom of choice for the vapor sources. A conditioning gas,such as a reducing gas, can be added to the carrier gas 26.Alternatively, a background pressure of a conditioning gas can beestablished in the vacuum chamber. Conditioning gases suitable for usewith the present invention include, but are not limited to, oxygen,argon, helium, hydrogen, nitrogen, halogen-containing compounds andmixtures thereof.

In embodiments in which the conditioning gas is included in a gas beam,a preferred combination of carrier gas and conditioning gas is argonwith approximately 4 to 10% hydrogen. In some embodiments, the hydrogencan be added downstream of the source 28 via a separate source.Alternatively, a conditioning gas such as the hydrogen can be suppliedin a separate cleansing stream which contacts the substrate materialbefore the substrate material is contacted by the layer-forming streamcontaining carrier gas 26 and the layer forming gas. The concentrationof the hydrogen is varied according to the specific materials selectedfor the epitaxial buffer layer 44 and according to the substratematerial.

In embodiments in which a background pressure of a conditioning gas isused, the hydrogen should comprise at least about 0.5 percent of thepressure of the chamber. Preferably, the hydrogen comprises from about0.5 percent to about 10 percent of the pressure of the chamber, morepreferably from about 1 percent to about 8 percent, and most preferablyfrom about 2 percent to about 6 percent.

The addition of a conditioning gas such as hydrogen or oxygen, either tothe carrier gas 26 or via a separate stream creates a reactivebackground for the layer forming gas which stimulates the nucleation ofan epitaxial layer on the surface 42 of the substrate 40. As notedabove, this reactive background can be established, at least in part, byestablishing a background pressure of a conditioning gas. In onepreferred embodiment, a conditioning gas may be supplied in thelayer-forming stream with the layer forming gas, or subsequently in aseparate stream, to promote formation of an interlayer for the desiredmaterial. In a preferred embodiment, the conditioning gas is leaked intothe chamber using a leak valve. For the case of a substrate where anepitaxial native oxide scale is preferred as an interlayer for theepitaxial layer, the conditioning gas may be oxygen, and an oxygenpartial pressure can be selected to form a desired oxide from elementswithin the substrate alloy. For example, oxygen partial pressure on theorder of 1×10⁻⁵ torr in the low vacuum carrier gas may be selected toform a stable epitaxial alumina or chromia scale on nickel base alloyswhile avoiding the formation of nickel oxide.

In another preferred embodiment, a background partial pressure of aconditioning gas reacts with a surface contamination of adsorbedelements, such as carbon or sulfur, forming a second gas phase which iscarried away from the substrate surface in the flowing gas stream. Thiseffect is emphasized in the presence of a native oxide scale on ametallic substrate surface. Native oxide scales can form in amorphousfashion, as random crystallographic scales or as epitaxial scales. Theepitaxial scales often form in a different crystallographic orientationfrom the substrate due to changes in crystal lattice parameter as theoxide is formed. These oxide scales can interfere with the nucleation ofepitaxially deposited oxide films from the desired metallic template.Adsorbed contaminants are cleaned from the surface of metallicsubstrates by forming a native oxide scale and then subsequentlyremoving that scale by reduction in high temperature in an atmosphere ofargon gas with a partial pressure of hydrogen. The present inventionallows for this cleansing process to take place in situ with the desiredepitaxial film deposition process. This invention also allows forprecise control over the presence, controlled growth or removal ofnative oxide films by adjustment of background reactive gas compositionin situ with the deposition process. In some embodiments, an additionalconditioning gas which can react with the layer forming gas may besupplied in the layer-forming stream with the layer forming gas, orsubsequently in a separate stream, to promote formation of the desiredmaterial. In a preferred embodiment, multiple separate directed vaporstreams containing a series of conditioning gases and layer forminggases can be employed to sequentially cleanse contaminants and nativeoxide films from the substrate surface and to deposit the desiredepitaxial oxide layer without removal from a high vacuum chamber orchanges in source material.

The carrier gas 26 and the layer forming gas form a dispersion which isaccelerated by the nozzle 28 to a positive velocity, which is greaterthan 1 m/sec and may be in the range of several hundred meters persecond. The nozzle 28 imparts a given velocity and kinetic energy to theatoms or molecules 36 of the layer forming gas and the gas 26. Thisdispersion forms a directed vapor stream 34 which is directed throughthe low vacuum environment 32 and toward the substrate 40. The kineticenergy of a given individual atom or molecule 36 is dependent upon thetype of material and the initial velocity produced by the source 28.

The low vacuum environment within the substrate deposition chamber ispreferably maintained at a pressure of at least about 1×10⁻³ torr,usually from about 1×10⁻³ torr to about 2 torr, but typically at lessthan 50 percent of the source chamber pressure in order to ensureadequate vapor stream acceleration to the substrate. It has beendiscovered that a low vacuum process of the present invention providesimproved results over conventional high vacuum processes. Inconventional physical vapor deposition, increases in deposition chamberpressure result in more collisions of the layer forming gas with gasmolecules, thereby reducing the mean free path of flight of the atoms ormolecules of the layer forming gas, and dispersing the vapor. Thepresent invention takes advantage of this otherwise detrimental effectto slow the individual atoms or molecules 36 of the layer forming gas toa lower velocity just above or at the substrate surface, and hence lowerthe kinetic energy level. This allows the interfacial mobility of thedepositing atoms or molecules to be high, allowing them to settle into astable low energy configuration upon deposition onto the substrate 40.This avoids random implantation of individual atoms 36 of the dispersiononto the substrate 40 at high velocity and high energy. The drivingforce for epitaxial nuclei to form is thus increased, while providing auniform, high rate-deposited layer 44 on the substrate surface 42 atlower temperatures than could otherwise be utilized.

Substrates for use in the process of the present invention 10 arepreferably cubic textured alloys or metals. However, single crystaloxide substrates and single crystal metal and alloy substrates have beenused successfully. Those skilled in the art will recognize many suchsubstrates which can be advantageously used under the scope of thepresent invention. A preferred substrate for use in a superconductorcomposite is a nonmagnetic nickel alloy with a cube texture.Additionally, other face centered cubic metals and alloys can bepreferably employed as substrate materials under the present invention.The use of the invention allows the deposition of epitaxial oxide bufferlayers on copper or copper-based alloys, such as copper-nickel withoutdeleterious oxidation of the metal or alloy substrate. More generally,any crystallographically oriented or textured substrate may be used,provided conditions suitable for epitaxy such as lattice match andchemical compatibility may be arrived at.

Examples of nickel-copper alloys are disclosed in commonly assigned U.S.patent application Ser. No. 08/943,047, entitled "Substrates withImproved Oxidation Resistance", filed Oct. 1, 1997, which is herebyincorporated by reference.

As shown by FIG. 3, the deposition and growth of epitaxial layers areenhanced by the removal of native oxide and surface contaminant 85 onthe substrate surface 42. The conditioning gas in the high energydeposition stream 34 disrupt the native oxide 37 on the surface 42 ofthe substrate 40. This exposes the crystallographically orientedsubstrate template 87 while removing species 85 and allows directdeposit of the layer forming gas onto the substrate surface 42 to forman epitaxial layer 44. In this manner, the crystallographic orientationof the substrate is transferred to the epitaxial layer. Thiscrystallographic orientation is then, likewise transferred to asuperconducting oxide when a superconducting composite is formed.

Referring to FIG. 4, the deposition of the layer forming gas onto thesurface 42 of the substrate 40 can be described in greater detail. Asthe stream 34 is directed at the substrate surface 42, the carrier gasforms a boundary layer 50 on the substrate 40. The boundary layer 50 iscreated by the carrier gas striking the surface of the substrate 40 atpositive velocity and bouncing back upward toward the source 28.However, since the stream 34 continues to flow downward towards thesubstrate 40 at a positive velocity, the reflected carrier gas is forceddownward again to the substrate, thereby forming a continuousoscillation and boundary layer above the substrate surface 42. Thegreater the velocity of the initial stream, the more significant theboundary layer effect. The boundary layer 50 assists in creatinginterference to change the velocity and hence, kinetic energy ofindividual atoms or molecules 36 of the layer forming gas from thehigher energy and higher velocity to a significantly lower velocity andkinetic energy prior to deposition of the individual atom or molecule 36onto the substrate surface 42. Thus, regardless of the initial speed ofthe stream, within the limits disclosed, the velocity and kinetic energyof the atoms or molecules 36 will be low at the substrate surface. Tofurther illustrate this point, reference is made to individual atoms 54,56, 58 and 60 of FIG. 4. Individual atom 54 is directed at the substrate40 at near-sonic velocity and high kinetic energy by source 28 andstrikes the boundary layer 50 above the substrate 40. The boundary layer50 causes interference with the individual atoms, such as atom 56, whichis slowed and reduced from relatively high energy and velocity to a muchlower velocity relative to the substrate normal. An atom which has beenslowed and interfered with by the boundary layer 50, such as atom 58, isthen free to float or settle into a stable low energy configuration onthe substrate surface 42. Once an atom is deposited on the substrate 40,such as atom 60, the elevated temperature of the substrate 40 allowsinterface mobility of the atom 60. This allows atom 60 to move on thesurface 42 to a stable position, in contrast to other high energydeposition processes which embed atoms into a substrate surface. Asseveral atoms are deposited on the surface and position themselves atdesired low energy configurations, an epitaxial layer 44 is grown.Epitaxial layer 44 can reach thicknesses of 0.01 to 5 micrometers withthe process of the present invention 10. Further, the temperature of thesubstrate 40 should be maintained at 25° C. to 800° C. for depositionand interface mobility.

This is a significant improvement over other deposition processes, suchas sputtering, which do not have the necessary low substrate arrivalenergy for individual atoms of the layer forming gas. A high arrivalenergy causes the atoms 36 to shoot off randomly in a static environmentwithout the carrier gas creating a boundary layer. By the process of thepresent invention, material usage efficiencies can be increased fromless than 50 percent to over 95 percent as materials are depositedwithout scattering. Materials are also not embedded into substratesurfaces with the present invention, thereby allowing a stable anduniform epitaxial layer to grow.

Referring now to FIG. 6, there is shown a superconducting compositearticle formed according to the processes of the present invention. Thesuperconducting composite 110 includes the substrate 112, an epitaxialbuffer layer 114 deposited on the substrate 112 and a superconductingoxide layer 116 deposited on the epitaxial layer 114. Epitaxial bufferlayers 114 can be deposited on the substrate 112 primarily to provide asuitable template for growth of the high temperature superconductor film116 and to provide a barrier to diffusion of substrate elements into thehigh temperature superconductor film 116. To be effective, the bufferlayer should be uniform, continuous and smooth. These features can beachieved with the process of the present invention. A defined epitaxialrelationship with the substrate 112 gives control of thecrystallographic template for high temperature superconductor filmnucleation and growth. Epitaxial buffer layers suitable for use with thesuperconductor produced according to the process of the presentinvention may include any of the materials listed previously and arepreferably deposited in thicknesses of 0.01 micrometers to 0.5micrometers. Orientation relationships between these buffer layermaterials and a given substrate material can be determined individually.

The superconducting oxide layer 116 is preferably a rare earthsuperconducting oxide, with YBa₂ Cu₃ O_(x) (YBCO) being preferred.Partial or complete substitution for yttrium, if desired, of neodymium,samarium, europium, gadolium, terbium, dysoberium, holmium, erbium,thulium, ytterbium, or lutetium can be made. Mercury and thallium basedhigh temperature superconductor films may also be used. The YBCO filmsare deposited on the textured substrate through a variety of methods,such as pulsed laser deposition, chemical vapor deposition, electronbeam evaporation, metallorganic or sol-gel solution precursor methods,as well as the process of the present invention can be utilized.

A preferred precursor approach uses a metallorganic triflouroacetateprecursor solution. With this method, high temperature superconductorfilms are spun or dip or web coated and then reacted to form thesuperconducting YBCO phase. The as-coated precursor includes anoxyflouride film containing BaF2. Heat treatment in a controlledatmosphere, such as that disclosed in U.S. Pat. No. 5,231,074 issued toCima et al. fully incorporated herein by reference, decomposes the BaF₂phase and thereby crystallizes the film. This allows the nucleation andgrowth of an epitaxial YBCO film. Alternatively, the precursor may bedeposited by e-beam evaporation or other evaporation or spray processesand subjected to the same controlled reaction process. YBCO filmthicknesses of approximately 1 micrometer to 10 micrometers arepreferred, and more preferably 2 micrometers to 5 micrometers. Thesuperconductor composite 110 preferably has a critical current densityof 1 MA/cm² to approximately 3 MA/cm².

In alternate embodiments, superconductor oxides can be formed using thegas phase methods of the invention. In addition, the gas phase methodsof the invention can be used to grow an epitaxial layer of asuperconductor directly onto the surface of a metal single crystal.

An epitaxial layer formed by the gas phase methods of the invention canhave a defect density that is at least about ten times less than thedefect density of epitaxial layers formed by known PVD, CVD andsputtering techniques. Moreover, epitaxial layers formed by the gasphase techniques of the invention can have porosities that are fromabout four to about five times less than the porosity of epitaxiallayers formed by known PVD, CVD and sputtering methods.

Pores present at the surface of an epitaxial layer represent one type ofimperfection in the epitaxial layer. Thus, it is desirable to provideepitaxial layers having a low pore density. Preferably, the epitaxiallayers have a pore density of less than about 500 pores per squaremillimeter, more preferably less than about 250 pores per squaremillimeter, and most preferably less than about 130 pores per squaremillimeter.

Inclusions present at the surface of an epitaxial layer can initiatenon-epitaxial growth, so it is advantageous to reduce the size of theseinclusions. Preferably, the epitaxial layers have inclusions with anaverage particle size diameter of less than about 1.5 micrometers, morepreferably less than about 1 micrometer, and most preferably less thanabout 0.5 micrometers.

While the above discussion has focused on embodiments in which multiplenozzles are used in series, in some embodiments, more than one nozzlecan be used in parallel. FIG. 7 illustrates such an embodiment in whichvacuum apparatus 200 includes beam sources 202 and 204 and surface 206which can be similar to the corresponding components discussed above.The layer forming gas can emanate from one beam source, and theconditioning gas can emanate from the other beam source. The substratesurface can be exposed to the conditioning gas and the layer forming gasin series or in parallel, as described above.

EXAMPLE I

The effect of substrate temperature on epitaxial layer growth wasstudied for epitaxial layers of cerium oxide formed on a single crystallanthanum aluminate substrate.

Cerium was sputtered from a metallic cerium target to form cerium vapor.The cerium vapor was entrained in a mixture of argon/10% oxygen gaswhich was directed toward the substrate at high velocity through anozzle. Thus, the carrier gas, layer forming gas and conditioning gaswere argon, cerium vapor and oxygen, respectively. In this example, theconditioning gas reacted with the layer forming gas to provide thecerium oxide layer.

The substrate was a polished LaAlO₃ single crystal chip with the {001}sheet normal face exposed to the deposition stream. The lineardeposition rate was approximately 12 nm/min. The substrate temperatureduring deposition was varied between 200° C. to 450° C. The depositionchamber pressure was maintained at about 120 millitorr.

A substrate temperature of less than about 350° C. resulted in CeO₂layers of relatively poor quality; in one case a {111} sheet normaltexture with random in-plane texture, in a second case a {001} sheetnormal texture with an in plane texture of approximately 15 degrees fullwidth half maximum on a {111} peak.

A substrate temperature of at least about 400° C. showed excellentepitaxial relationship with the LaAlO₃ substrate, with full widths athalf maximum ranging from 5 degrees to 7.7 degrees.

Separate experiments on deposition of CeO² on LaAlO³ were conducted inan electron-beam chamber. CeO₂ could not be deposited epitaxially from ametallic source but could be deposited epitaxially from an oxide source.

The use of a metallic source was made possible in the low vacuumapproach by the ability to add a layer forming gas, oxygen, to thecarrier gas stream. Evaporation rates are generally higher from metallicsources, increasing the viability of the low vacuum process for aneconomical manufacturing capability.

EXAMPLE II

The effect of using removing a native oxide from the substrate surfacewas studied.

Cerium vapor was formed as in Example I. The substrate was a polishedsingle crystal nickel disk oriented with the {001} sheet normal. Thevapor was entrained in a gas mixture of argon or argon plus 5-10%oxygen.

The substrate temperature was raised in a series of experiments from the400° C. to 540° C. Epitaxial layers were grown in each case.

All samples exhibited a mixed {001}/{111} sheet normal texture in thedeposited film. The amount of {001} texture as measured by relativeX-ray peak intensities increased with increasing temperature, howeverthe {111} texture was never eliminated. A representative pole figure isshown in FIG. 8. Both sheet texture variants appeared to be formedepitaxially at the highest temperature since very little random in-planeorientations were observed. The full width at half maximum for the {001}variant improved from over 45 degrees to less than 30 degrees withincreasing temperature, indicating that higher temperatures may berequired to obtain the best quality films. Further efforts to produce asingle {001} oriented epitaxial film by temperature increases werecurtailed since the results indicated that temperature alone could noteliminate the {111} orientation.

The naturally occurring nickel oxide on a {001} oriented nickel singlecrystal is the {111} orientation. These results indicated that theelimination of this oxide was advantageous in allowing reliabledeposition of {001} oriented epitaxial films on the nickel substrate.

EXAMPLE III

The effect of controlling native oxides at the substrate surface wasstudied.

A series of experiments were performed to evaluate the combined effectsof temperature and the use of hydrogen to control the formation ofnative surface oxides and subsequent epitaxial growth of oxide films onmetallic substrates.

A cerium metal sputtering target was used as the source for the ceriumvapor. The substrate was a polished, single crystal nickel with a {001}surface normal. The temperature was varied from 500° C. to approximately700° C. The hydrogen, which acted as a conditioning gas in the gas beam,content of the argon carrier gas was varied from 2% to nearly 10%. Thehydrogen addition was eliminated and replaced with a small amount ofoxygen to ensure formation of the stable cerium oxide once an initialoxide film layer was deposited (nucleated) on the metal surface.

The amount of {001} CeO₂ texture increased relative to the {111} texturewith the addition of 2% hydrogen as temperature increased from 500° C.to 650° C., though the {111} texture was not eliminated. A single, fullyepitaxially oriented {001} CeO₂ film was produced at 650° C. withhydrogen contents of about 4% and about 8%. This established that lowerthresholds for both temperature and reactive gas (hydrogen) contentexist for the successful high rate deposition of epitaxial oxide filmson metallic substrates without requiring conditioning of the substratesurface and growth of the epitaxial layer in series. Full widths at halfmaximum for the CeO₂ films ranged from 6.8 degrees to 11 degrees attemperatures and hydrogen pressures above the epitaxial threshold (inthis case 650° C. at 4% hydrogen, a temperature identical to itsepitaxial temperature in a prior art high vacuum process).

EXAMPLE IV

Epitaxial layers were grown on polycrystalline substrates as follows.

Cerium oxide epitaxial films were deposited following the proceduresestablished in Example III with the exception that the substrates weredeformation textured nickel. The full width at half maximum of thenickel substrate was approximately 13 degrees. The epitaxial CeO₂ filmfull width at half maximum was approximately 18 degrees, and its polefigure is shown in FIG. 9. This demonstrated the utility of thedeveloped process for production of tapes.

A YBa₂ Cu₃ O_(x) film was deposited on a representative CeO2 buffered,deformation textured nickel tape by spin coating from a metallorganicprecursor solution. The film was reacted by controlled heat treatment toform a superconducting oxide film.

EXAMPLE V

An epitaxial layer of cerium oxide was grown on a nickel substrateaccording to the procedure of Example III using a nickel substratetemperature of about 650° C. An epitaxial layer of yttria-stabilizedzirconia was grown on the cerium oxide layer at a surface temperature ofabout 700° C. FIGS. 10A-10C show x-ray pole figures of the nickelsubstrate, the cerium oxide/nickel article and the yttria-stabilizedzirconia/cerium oxide/nickel article, respectively. The full widths athalf maximum for these figures were 10°, 12° and 10°, respectively.

Micrographs of the epitaxial layer demonstrated a pore density of 130pores per square millimeter. The micrographs also demonstrated anaverage second phase particle size of 0.5 micrometers.

EXAMPLE VI

A cerium oxide/yttria-stabilized zirconia/cerium oxide buffer layerstack was formed on a textured nickel substrate by passing argon througha deposition source and hydrogen through a second source such that thepartial pressure of hydrogen in the chamber was from about 2 percent toabout 4 percent of the total chamber pressure. After an initialtreatment of the argon/hydrogen mixture to remove the nickel oxide scaleon the substrate, the initial cerium oxide layer was deposited.Yttria-stabilized zirconia was then deposited with argon again passingthrough the deposition source and oxygen passing through another sourcesuch that the partial pressure of oxygen in the chamber was from about 2percent to about 4 percent of the total chamber pressure. A cerium oxidecap layer was then formed in the same manner as the yttria-stabilizedzirconia layer, with argon passing through the deposition source andoxygen passing through the other source. The substrate temperatureduring the formation of all three layers was from about 500° C. to about750° C.

A θ-2θ scan showed peak identifications as follows:

    ______________________________________                                          Material                                                                             Orientation   Angle   Intensity                                      ______________________________________                                          Ni     200           51.89   >8000                                          YSZ                  200                                                                                               7800                                 YSZ                  111                                                                                                 400                                CeO.sub.2                                                                                     200                       2000                                CeO.sub.2                                                                                     111                       20                                  ______________________________________                                    

EXAMPLE VII

A superconductor layer was deposited on the ceriumoxide/yttria-stabilized zirconia/cerium oxide buffer layer of Example V.

The superconductor layer was formed according to the methods disclosedin U.S. Pat. No. 5,231,074 using a trifluoroacetate precursor. Theprecursor film was spin-coated on a six millimeter by 6 millimeterbuffer substrate and decomposed at 400° C. to form a BaF₂ -containingsolid film. The film was heat treated at about 785° C. in 0.1 percentoxygen (balanced by nitrogen). The heat treatment was carried out in thefollowing steps. The first step was performed under a low water vaporpressure of about 0.6 percent relative humidity (22° C.). This stepstarted from 100° C. to ten minutes after reaching 785° C. The secondstep was performed under a high water vapor pressure of 100 percentrelative humidity (22° C.). The second step started after 10 minutes at785° C. and lasted two hours. The third step involved purging with dry0.1 percent oxygen for ten minutes at 785° C. before cooling down to435° C., at which the atmosphere is changed to pure oxygen and held forthree hours and then slowly cooled down to room temperature.

This procedure resulted in a superconductor having a transitiontemperature as shown in FIG. 11. The superconducting transitiontemperature was measured using a standard 4-point measurement technique.As shown in FIG. 11, the superconducting transition was clear.

The chamber pressure during growth of the cerium oxide layer and theyttria-stabilized zirconia layer was about 0.1 torr. The temperaturesused during the growth of the cerium oxide and yttria-stabilizedzirconia layers was comparable to those used for electron beamdeposition at a pressure of 1×10⁻⁵ torr. The growth rate achieved forthe cerium oxide and yttria-stabilized zirconia layers was about fivetimes higher than typically achieved at a pressure of about 1×10⁻⁵ torr.

Other embodiments are within the claims.

What is claimed is:
 1. A method of forming an epitaxial layer of adesired material chemically comprising a first species on a depositionsurface of a substrate chemically comprising one or more substratespecies including at least one species different from the first species,at a predetermined deposition rate, comprising the steps of:placing acrystallographically oriented target surface of a substrate in a lowvacuum environment, the species present in the target surfacesubstantially consisting of the substrate species, and one or morematerials selected from the group consisting of desired native oxides ofone or more of the substrate species, and contaminant materials, whichinclude undesired native oxides of one or more of the substrate speciesand adsorbed surface contaminants; heating the target surface to atemperature which is less than about 90% of the melting point of thesecond material but greater than the threshold temperature for formingan epitaxial layer of the desired material on the substrate material ina high vacuum environment at the predetermined deposition rate;directing a layer-forming stream comprising a dispersion of the firstspecies in an inert carrier gas at a first velocity greater than about 1m/sec toward said target surface through said low vacuum environment;directing a dispersion of a beneficially reactive second species at avelocity greater than about 1 m/sec toward said target surface throughsaid low vacuum environment at a velocity substantially similar to saidfirst velocity; reacting said second species with one or more of thespecies present in the target surface to condition the target surfaceand promote the nucleation of the epitaxial layer; depositing a desiredmaterial chemically comprising said first species onto said targetsurface at the predetermined deposition rate to form an epitaxial layeron said substrate; and depositing an oxide superconductor on saidepitaxial layer.
 2. The method of claim 1 further comprising the step ofreducing said first velocity to a second lower velocity by forming aboundary layer of said carrier gas at said target surface.
 3. The methodof claim 1 wherein the target surface is heated to a temperature whichis less than the threshold temperature for forming an epitaxial layer ofthe desired material on the substrate material in the low vacuumenvironment at the predetermined deposition rate.
 4. The method of claim1 wherein said substrate material is metallic.
 5. The method of claim 4wherein said substrate material is a cubic textured alloy.
 6. The methodof claim 4 wherein said second species is selected to be reducing withrespect to said undesired native oxides and said reaction step is areducing reaction which at least partially removes said undesired nativeoxides from said target surface.
 7. The method of claim 4 wherein saiddesired material is an oxide of the first species and said secondspecies is selected to be more oxidizing with respect to said firstspecies than with respect to the substrate material thereby reducingundesired native oxides on the target surface while allowing formationof an epitaxial oxide layer from said first species.
 8. The method ofclaim 4 wherein said desired material is metallic and said secondspecies is selected to be reducing with respect to said undesired nativeoxides, said substrate material and said desired material thereby atleast partially removing said undesired native oxides from said targetsurface while allowing formation of a metallic epitaxial layer from saidfirst species.
 9. The method of claim 1 wherein said substrate materialis an oxide.
 10. The method of claim 1 wherein said second species isselected to be reactive with respect to said adsorbed surfacecontaminants thereby at least partially removing said trace surfacecontaminants from said target surface.
 11. The method of claim 1 whereinthe desired material consists essentially of the first species.
 12. Themethod of claim 1 wherein said desired material consists essentially ofthe reaction product of the first species and the second species. 13.The method of claim 1 wherein said second species is supplied in saidlayer-forming stream.
 14. The method of claim 1 wherein said secondspecies is supplied in a cleansing stream which contacts the substratematerial before said substrate material is contacted by saidlayer-forming stream.
 15. The method of claim 14 further comprising thestep of supplying a third species reactive with said first species. 16.The method of claim 15 wherein said desired material consistsessentially of the reaction product of the first species and the thirdspecies.
 17. The method of claim 15 wherein said third species issupplied in said layer-forming stream.
 18. The method of claim 15wherein said third species is supplied in a separate stream after saidsubstrate material is contacted by said layer-forming stream.
 19. Themethod of claim 1 further comprising the step of supplying the firstspecies to the stream from a metallic vapor source.
 20. The method ofclaim 19 wherein said desired material is an oxide and said substratematerial is metallic, and wherein said first species is selected to bemore noble than said substrate material, thereby promoting bothreduction of said undesired native oxides and formation of said desiredmaterial.
 21. A composite article made according to the method ofclaim
 1. 22. A method of making a superconductor, the methodcomprising:growing an epitaxial buffer layer on a substrate having atemperature that is about the same as a PVD epitaxial growth thresholdtemperature for a chamber pressure of about 1×10⁻⁴ Torr, wherein thegrowing step is performed in a chamber having a pressure of at leastabout 1×10⁻³ Torr; and depositing on the epitaxial buffer layer a layerof material selected from the group consisting of superconductormaterials and precursors of superconductor materials.
 23. The method ofclaim 22 wherein said step of depositing an oxide superconductorincludes depositing a precursor and converting said precursor to formsaid oxide superconductor.
 24. The method of claim 23 wherein saidprecursor includes an organometallic trifluoroacetate.
 25. The methodaccording to claim 22, wherein the substrate has a temperature of fromabout 25° C. to about 800° C.
 26. The method according to claim 22,wherein the substrate has a temperature of from about 500° C. to about800° C.
 27. The method according to claim 22, wherein the substrate hasa temperature of from about 500° C. to about 650° C.
 28. The methodaccording to claim 22, wherein the chamber has a pressure of at leastabout 0.1 torr.
 29. The method according to claim 22, wherein thechamber has a pressure of at least about 1 torr.
 30. A method of makinga superconductor, the method comprising the steps of:growing anepitaxial buffer layer on a substrate at a rate of at least about 50Angstroms per second; and depositing on the epitaxial buffer layer alayer of material selected from the group consisting of superconductormaterials and precursors of superconductor materials.
 31. The methodaccording to claim 30, wherein the substrate has a temperature of fromabout 25° C. to about 800° C.
 32. The method according to claim 30,wherein the substrate has a temperature of from about 500° C. to about800° C.
 33. The method according to claim 30, wherein the substrate hasa temperature of from about 500° C. to about 650° C.
 34. The methodaccording to claim 30 wherein the chamber has a pressure of at leastabout 1×10⁻³ torr.
 35. The method according to claim 30, wherein thechamber has a pressure of at least about 0.1 torr.
 36. The methodaccording to claim 30, wherein the chamber has a pressure of at leastabout 1 torr.
 37. A method of making a superconductor, the methodcomprising the steps of:exposing a surface of a substrate to aconditioning gas that interacts with surface of the substrate to form aconditioned surface of the substrate; exposing the conditioned surfaceof the substrate to a gas beam having a layer forming gas that becomes acomponent of an epitaxial buffer layer on the conditioned surface of thesubstrate; and depositing on the epitaxial buffer layer a layer ofmaterial selected from the group consisting of superconductor materialsand precursors of superconductor materials.
 38. The method according toclaim 37, wherein the substrate has a temperature of from about 25° C.to about 800° C.
 39. The method according to claim 37, wherein thesubstrate has a temperature of from about 500° C. to about 800° C. 40.The method according to claim 37, wherein the substrate has atemperature of from about 500° C. to about 650° C.
 41. The methodaccording to claim 37 wherein the pressure of the chamber is at leastabout 10⁻³ torr during the exposing steps.
 42. The method according toclaim 37, wherein the pressure of the chamber is at least about 0.1 torrduring the exposing steps.
 43. The method according to claim 37, whereinthe pressure of the chamber is at least about 1 torr during the exposingsteps.
 44. A method of making a superconductor, the method comprisingthe steps of:growing an epitaxial superconductor layer on a surface of amaterial having a temperature that is about the same as a PVD epitaxialgrowth threshold temperature for a chamber pressure of about 1×10⁻⁴torr, wherein the growing step occurs in a chamber having a pressure ofat least about 1×10⁻³ torr, and wherein the material is a buffer layeror a substrate.
 45. The method according to claim 44, wherein thematerial is an epitaxial buffer layer.